It is known that tuners having two or more frequency converters may suffer from the generation of spurious heterodyning products appearing in the frequency band of the output signal and resulting from spurious non-linear mixing of two or more of the local oscillator signals. For example, a known type of double-conversion tuner comprises a first up-converter for converting the frequency of any selected channel to a high first intermediate frequency, such as 1220 MHz, by high-side mixing, i.e., the local oscillator in the first frequency converter is at a frequency above that of the selected channel. The channel at the high intermediate frequency is then down-converted in a second frequency changer to a second output intermediate frequency, such as 45.75 MHz, by low-side mixing, i.e., the local oscillator in the second frequency converter is at a frequency below the first intermediate frequency. Such a tuner may be used to select, from a broadband input signal containing a plurality of channels available for reception, a channel for example at a frequency of 751.25 MHz. The first local oscillator is thus set to a frequency of 1971.25 MHz to convert the selected channel to the first intermediate frequency of 1220 MHz. The second local oscillator is set to a frequency of 1174.25 MHz to convert the selected channel from the first intermediate frequency to the second or output intermediate frequency of 45.75 MHz.
The first and second local oscillators also produce outputs at harmonics, higher than the first harmonic or fundamental frequency, and such harmonics may mix non-linearly to produce spurious products within the actually utilised bandwidth at the second intermediate frequency, for example from 41 to 47 MHz. For example, the third harmonic of the first local oscillator will be at a frequency of 5913.75 MHz and the fifth harmonic of the second local oscillator will be at a frequency of 5871.25 MHz. Spurious heterodyning may result in a spurious product at a frequency of 42.5 MHz being present in the output signal, which is within the frequency spectrum actually occupied by the selected channel at the second intermediate frequency.
Another example of generation of a spurious heterodyne product within the output frequency band occurs when the above-described tuner selects an input channel at 157.25 MHz. In this case, the first local oscillator is tuned to a frequency of 1377.25 MHz and the second local oscillator is tuned to a frequency of 1174.25 MHz (as before). The sixth harmonic of the first local oscillator and the seventh harmonic of the second local oscillator are then at 8263.5 MHz and 8219.25 MHz, respectively, so that a spurious product can be produced at 43.75 MHz, which is within the utilised output bandwidth.
The actual frequencies at which such beats or products occur are dependent, among other factors, upon the first and second intermediate frequencies. Problems caused by such beats may therefore be overcome, as is known, by varying the first high intermediate frequency. For example, in the situation where a channel at 751.25 MHz is selected for reception, the first intermediate frequency may be changed to 1221 MHz; this is generally permissible because the bandwidth of any intermediate frequency filtering at the first high intermediate frequency is generally sufficiently wide to pass several channels so that a shift of the order of 1 MHz in the first intermediate frequency will not result in unacceptable performance of the tuner. In order to provide the shifted first intermediate frequency, the first local oscillator frequency is changed to 1972.25 MHz and the second local oscillator frequency is changed to 1175.25 MHz. The third harmonic of the first local oscillator is now at 5916.75 MHz whereas the fifth harmonic of the second local oscillator is now at 5876.25 MHz. The resulting heterodyne product is now at a frequency of 40.5 MHz and is thus outside the output bandwidth at the second intermediate frequency utilised by the selected channel.
A known “empirical” method for determining shifts in intermediate frequency to avoid problems with spurious heterodyne products is performed during a development phase of a tuner. During this phase, the desired channels are supplied to the tuner and the output spectrum is observed on test equipment, such as a spectrum analyser. If spurious output beats are observed, the first intermediate frequency is manually varied so as to remove the beat from the frequency band utilised at the output of the tuner. The variation is then recorded and written into control software for the tuner.
This process has to be performed for each channel available for reception by the tuner and thus is relatively time-consuming and prone to human error. Also, the resulting intermediate frequency variations or “dither pattern” will only apply to a given set of conditions, i.e., choice of the nominal first and second intermediate frequencies, the channel bandwidth utilised within the tuner and the frequency planning of the received spectrum.
Another disadvantage of this technique is that, if the received channels can vary in their frequency, then the empirically determined dither pattern may no longer apply. In practice, this may arise, for example, when a tuner of this type is used with a preconversion stage, as in the case of satellite receiver systems where the received channels are first down-converted by an external block converter to the tuner input frequency range. In this example, the external conversion has an absolute frequency accuracy or tolerance of ±5 MHz and the frequency of each channel may vary within these limits from the nominal channel frequency.
Another known “measurement” technique is used when a tuner is actually installed in a receiving system or apparatus. In this technique, the system is required to include arrangements for measuring spurious heterodyne products or beats together with, for example, control software for adjusting the first intermediate frequency in order to eliminate any beats which are measured. The control software may, for example, be provided within the digital signal processing of a digital demodulator.
Effectively, the same empirical method as described hereinbefore is performed during operation of a system including the tuner, for example whenever a change is requested to the channel selected for reception. However, this technique requires the presence of monitoring means for monitoring the presence of spurious products together with sophisticated software requiring processing time and memory allocation within a system controller. Also, the time required for performing this technique may be objectionably large to a user.
It is also known to provide tuner arrangements comprising two or more tuners, each of which is of the multiple conversion, such as double conversion, type. In the case of a dual tuner arrangement, there are four or more local oscillators which are independently adjustable. Such arrangements are required to be compact and all of the tuners present in the arrangement generally receive the same broadband input signal. There is therefore a high risk that all four or more oscillators will beat with each other to cause spurious beat products within the utilised output bandwidth of either or both tuners.
The above-described empirical technique may be used with such multiple-tuner arrangements. However, when empirically determining each dither pattern, the frequencies of the local oscillators of the other tuner or tuners must be taken into account as they can beat with the local oscillators of the tuner whose dither pattern is being determined.
The possible interaction between the tuners results in a substantial increase in the time required to perform the empirical technique. For example, in the case of a dual tuner arrangement where each tuner is capable of receiving any one of one hundred input channels, beat product measurement and intermediate frequency variation must be assessed for each combination of channels for each tuner. This results in there being twenty thousand combinations, all of which must be assessed by the empirical technique. This technique is therefore extremely time-consuming.
Furthermore, this technique can never be completely reliable. For example, even if a dual-tuner arrangement has been properly set up by the empirical method, problems can occur during use as follows.
When each of the tuners is tuned to select respective input channels for reception, there will be no spurious heterodyne products or beats in the utilised output bandwidths of the tuners. However, if a first of the tuners is then re-tuned to a different channel which requires “dithering” or variation of the first intermediate frequency in the second tuner, the second tuner cannot be re-tuned since this would cause a break in reception. The spurious product can therefore only be removed satisfactorily by varying the first intermediate frequency of the first tuner. The first tuner will then no longer be set to the first intermediate frequency appropriate for the channel being received so that the “beat dithering pattern” established empirically is no longer valid. When the second tuner is re-tuned to select a different channel, the inappropriate first intermediate frequency of the first tuner remains so that there is no longer any information, for the new operating mode of the two tuners, as to whether a beat exists so that no action can be taken if a spurious beat is present in the utilised output bandwidth of either tuner.
The measurement technique may also be applied to a multiple-tuner arrangement but is not wholly satisfactory. In order to describe this, the example of a dual-tuner arrangement will again be considered.
Assuming that both tuners are initially correctly aligned and that any spurious beats have been eliminated, if a first of the tuners is re-tuned to select a different channel and spurious beats exist in the utilised output bandwidth, such spurious beats may be detected and removed by varying the first intermediate frequency of the first tuner in response to the monitored outputs of both the first and second tuners. Although this process will remove spurious beats, such beats will be present in the channel selected by the second tuner for a period during which disruption in reception by the second tuner may occur.
EP1248360 discloses a double conversion tuner which uses a table containing offsets for first and second oscillator frequencies. This technique IS specifically concerned with spurs forming within the passband of the first intermediate frequency (IF) filter but only discloses varying the second oscillator frequency so that the second IF will also vary when an offset is required to avoid a spur. The table is formed during design of the receiver.
U.S. Pat. No. 6,567,654 provides a table for a set of different first intermediate frequencies, which table is calculated using a computer or the like. The table is then used to determine a further table of frequency synthesiser controlling words for first and second local oscillators to avoid spurs.
EP1187774 discloses two techniques for avoiding interference. In the first, a look-up table is formed during development. In the second, the local oscillator frequencies are offset dynamically while monitoring tuner performance so that offsetting is adjusted until acceptable performance is achieved.
U.S. Pat. No. 4,661,995 makes use of pre-calculated values to avoid spurs and to shift first and second local oscillator frequencies in order to provide a constant second IF.
DE10116880 discloses a technique for compensating for temperature shift in the centre frequency of a first IF filter. This is performed by measuring temperature and using this to determine frequency shifts for first and second local oscillators.